1. Field of the Invention
The present invention relates to implementing a wireless communication scheme which is robust even in a poor wireless propagation environment in a wireless communication system including wireless communication devices such as cellular phones and wireless LANs which perform transmission/reception of data through wireless medium.
2. Description of the Related Art
According to wireless LAN specification IEEE 802.11e which has realized enhancement associated with the Quality of Service (QoS) of a Medium Access Control (MAC) layer with respect to the IEEE 802.11 standard specifications, as methods of acquiring a transmission opportunity (TXOP) period during which a transmitting-side communication apparatus (initiator) can transmit data, the Enhanced Distributed Channel Access (EDCA) scheme and the HCF Controlled Channel Access (HCCA) scheme are available. (See IEEE 802.11e Draft 13.0 and IEEE P802.11e/D13.0, January 2005.)
In IEEE 802.11n directed to faster transmission, a plurality of aggregation methods such as Aggregated-MAC Protocol Data Unit (A-MPDU) and High-throughput PHY (HTP) burst have been proposed to reduce the overhead existing between the respective frames in transmitting/receiving operation in IEEE 802.11e.
According to A-MPDU, an aggregation frame obtained by combining a plurality of MAC frames with one Physical Layer (PHY) frame with one field identifying each MAC frame being attached to the head of each frame is transmitted. (See TGn Sync Proposal Technical Specification, IEEE 802.11-04/889r6, May 2005.)
In HTP Burst, PHY frames are transmitted at intervals of a Reduced Interframe Space (RIFS), which is shorter than a Short Interframe Space (SIFS) period used for a conventional burst transmission technique. According to HTP Burst, when frames are to be transmitted to a plurality of receiving-side communication apparatuses (responders) at different transmission rates or with different transmission powers, the RIFS time is provided between the respective PHY frames to transmit the respective PHY frames at different transmission rates or with different transmission powers. (See TGn Sync Proposal Technical Specification, IEEE 802.11-04/889r6, May 2005, and WWiSE Proposal: High throughput extension to the 802.11 Standard, IEEE 802.11-05/0149r2, March 2005.)
In IEEE 802.11n, there has been proposed a method of improving transmission efficiency by a technique of performing bi-directional communication based on a piggyback technique during a TXOP time, acquired by an initiator, by making the initiator which has acquired the TXOP time allocate part of the TXOP time (TXOP allocation time) to a responder, i.e., a Reverse Direction (RD) scheme.
In IEEE 802.11n, when A-MPDU is used for the RD scheme (in which an initiator performs bi-directional communication with a responder by the piggyback technique during the TXOP time acquired by the EDCA scheme or the HCCA scheme), the initiator transmits an Initiator Aggregation Control (IAC) frame, and the responder returns a Responder Aggregation Control (RAC) frame the SIFS time after the transmission of the frame, thus performing IAC-RAC frame exchange. If the RD scheme is used on the assumption that such IAC-RAC frame exchange is performed, the initiator transmits, to the responder, an IAC frame in which information indicating the use of the RD scheme during an acquired TXOP time is written. Upon receiving the IAC frame and being notified of the information indicating that the RD scheme is used for communication in the TXOP time, the responder transmits an RAC frame addressed to the initiator after writing, in the frame, the number of data frames which the responder can transmit when part of the TXOP time is allocated, and a transmission data rate. The initiator determines an Reverse Direction Grant (RDG) duration as part of the TXOP time to be allocated to the responder from the number of data frames and the transmission data rate which are written in an RAC frame. The initiator writes the determined RDG duration in the IAC frame, attaches the IAC frame to the head of an aggregation frame to be transmitted, and transmits the aggregation frame the SIFS time after the completion of the reception of the previous RAC frame.
In this case, a data frame acknowledgement method (AckPolicy) is a BlockAck scheme. If the immediate BlockAck scheme (in which upon receiving an acknowledgement request frame (BlockAck request frame), the responder transmits an acknowledgement frame (BlockAck frame) after the lapse of the SIFS time) defined in IEEE 802.11e is used as this BlockAck scheme, a BlockAckRequest frame is also combined with the end of an aggregation frame to be transmitted from the initiator. (Note, however, that in the Implicit Block Ack scheme proposed in IEEE 802.11n, BlockAckRequest is omitted.)
In the above case, when the SIFS time elapses after the reception of the aggregation frame from the initiator, the responder must transmit receiving statuses through a block Ack frame. In the RD scheme, when a block Ack frame is to be returned from the responder after the lapse of the SIFS time, the responder transmits an aggregation frame which is combined with a plurality of data frames and a block Ack frame like the piggyback technique. The time taken for the transmission of this aggregation frame must not exceed the RDG duration written in the IAC frame. When requesting an RDG duration in transmitting an aggregation frame, the responder inserts, in an RAC frame, the number of data frames ready for transmission (i.e., frames scheduled to be transmitted this time) and a transmission data rate, and returns the frame upon attaching it to the head of an aggregation frame to be transmitted this time. (See TGn Sync Proposal Technical Specification, IEEE 802.11-04/889r6, May 2005.)
In the above RD scheme, however, since a BlockAck frame and a BlockAckRequest frame are combined with data frames to be transmitted as one PHY frame, the data frames, the BlockAck frame, and the BlockAckRequest frame are transmitted at the same transmission rate. For this reason, the probability of transmission errors due to a deterioration in the wireless propagation environment or the occurrence of collisions becomes almost the same as in the data frames, the BlockAck frame, and BlockAckRequest frame.
In general, since the transmission error probability increases when a high transmission rate is used, the transmission rate of an aggregation frame needs to be decreased to increase the transmission success probabilities of a BlockAck frame and BlockAckRequest frame. Decreasing the transmission rate, however, will increase the transmission length of an aggregation frame, resulting in a decrease in throughput.